This invention relates to an electroless copper plating solution mainly used for a formation of circuits in electronic parts, an electroless copper plating process and process for producing a circuit board each using said electroless copper plating solution; and particularly to a plating solution and plating technique each using glyoxylic acid but not using formaldehyde having a high volatility as a reducing agent for copper ion.
In JP-A-7-268638, there is proposed a plating process which comprises carrying out a plating while filtering a plating solution with the aim of preventing a body to be plated from a deposition of sodium oxalate precipitate formed in an electroless copper plating solution.
In JP-A-61-183474, there is mentioned a technique of using glyoxylic acid as a reducing agent in electroless copper plating solution. In this document, it is mentioned that NaOH or KOH is used for alkalifying the pH of electroless copper plating solution, and especially that KOH is more desirable than NaOH because potassium oxalate is higher than sodium oxalate in solubility of an oxalate salt which is an oxidized product of glyoxylic acid.
An electroless copper plating solution usually comprises copper ion, a complexing agent for copper ion, a reducing agent for copper ion and a pH adjusting agent.
As the reducing agent for copper ion, formaldehyde, glyoxylic acid or a salt of glyoxylic acid has been used generally. In plating solutions, an ion of oxidized product of reducing agent accumulates. In the case of using formaldehyde as a reducing agent for copper ion, the accumulating substance is formate ion. In the case of using glyoxylic acid as the reducing agent, the accumulating substance is oxalate ion.
On the other hand, as the pH adjusting agent in electroless copper plating solutions, NaOH has been generally used.
When NaOH was used as a pH adjusting agent and glyoxylic acid was used as a reducing gent, there has arised a difficulty that sodium oxalate is so small in solubility that precipitate of sodium oxalate was formed in the plating solution in the way of plating process. If such a solid precipitate is deposited on a body to be plated, no plating film can be formed in the area to which the precipitate is attached, and there is formed the so-called xe2x80x9cvoidxe2x80x9d.
In the case of using glyoxylic acid, the accumulation of oxalic acid in a plating solution is caused not only by the plating reaction but also by the Cannizzaro reaction. In the case of using glyoxylic acid, the Cannizzaro reaction is expressed by the following scheme:
2CHOCOOH+2OHxe2x88x92xe2x86x92C2O42xe2x88x92+HOCH2COOH+H2O
Since reaction rate of this reaction increases with elevation of temperature of plating solution, the progress of Cannizzaro reaction can be suppressed by keeping the plating solution at a low temperature. JP-A-2000-144438 discloses a plating apparatus equipped with a plating-practicing chamber and a circulation tank for circulating the plating solution. In this apparatus, the Cannizzaro reaction can be suppressed by always keeping low the temperature of plating solution present in the circulation tank for storing the plating solution.
Further, in this document, it is also mentioned that methanol can be added to the plating solution in some cases for the purpose of preventing the deterioration of plating solution caused by the Cannizzaro reaction, even though it is not expressly mentioned there whether the Cannizzaro reaction is that of formaldehyde or that of glyoxylic acid.
However, it is mentioned there that the method of adding methanol does not suppress the Cannizzaro reaction itself but the effect of this method has a certain limitation. That is, in the practical use of electroless copper plating solution, no successful case of suppressing the Cannizzaro reaction by the use of methanol has ever been known.
In xe2x80x9cHyomen Gijutsu (Surface Technology)xe2x80x9d, Vol.42, No.9, Pages 913-917 (1991) and The 6th national convention Record of Purinto Kairo Jissou Gakkai (Japan Institute of Printed Circuit), Pages 101-102, there is mentioned that, in the electroless copper plating solutions using glyoxylic acid as a reducing agent, the Cannizzaro reaction can be suppressed by using KOH as a pH adjusting agent more effectively than by using NaOH as a pH adjusting agent. Further, these reports refer to the higher solubility of potassium oxalate as compared with that of sodium oxalate, too.
In the case of these reports, nevertheless, NaOH was still used for realizing an alkaline pH value (pH=12.5) at the time of preparing the plating bath. As its result, precipitation of sodium oxalate corresponding to the quantity of sodium ion introduced into the plating solution in the initial stage was unavoidable in the case of these reports.
In the case of using glyoxylic acid as a reducing agent for the electroless copper plating solution, there have so far been problems that the reacting quantity of Cannizzaro reaction is larger, the plating solution is less stable, and the cost is higher than in the case of using formaldehyde as the reducing agent.
As to the stability of plating solution, it can be considered that a high reacting quantity of Cannizzaro reaction results in an elevation of salt concentration in the plating solution, which brings about a reduction of dissolved oxygen concentration in the plating solution and thereby an instability of plating solution.
Further, in the case of using glyoxylic acid, the progress of Cannizzaro reaction or the plating reaction brings about accumulation of oxalic acid, namely the oxidized product of glyoxylic acid, in the plating solution. In the prior electroless copper plating process, it has been conventional to carry out the plating process while adding NaOH to keep the plating solution alkaline. Thus, there has been a problem that, since sodium oxalate is low in solubility, the crystal of sodium oxalate precipitates in the plating solution and deposited on some area of base board, where no plating layer can be deposited and a void is formed.
With the aim of avoiding the rise in the salt concentration in the plating solution and the formation of precipitate of sodium oxalate, a method of adding KOH as a pH adjusting agent for keeping pH value alkaline during the plating treatment has been studied.
The description of xe2x80x9cHyomen Gijutsu (Surface Technology)xe2x80x9d, Vol.42, No.9, Pages 913-917 (1991) shows, however, that such a method of using KOH gave so low a Cannizzaro reaction-suppressing effect as 15-40%, as compared with the case of using NaOH. Although the suppressing effect was 40% as compared with NaOH when one hour had passed after start of the plating, the suppressing effect dropped to 15% when 5 hours had passed after start of plating (xe2x80x9cHyomen Gijutsu (Surface Technology)xe2x80x9d, Vol.42, No.9, Page 915, line 16 of the main text). Since electroless copper plating solution is usually used for a long period of time, the tendency of decreasing the suppressing effect with time is a fatal disadvantage.
Further, as is mentioned in xe2x80x9cHyomen Gijutsu (Surface technology)xe2x80x9d, Vol.42, No.9, Page 913-917 (1991) and JP-A-61-183474, potassium oxalate is higher than sodium oxalate in solubility and hence the use of KOH as a pH adjusting agent is considered advantageous from the viewpoint of preventing the precipitation of oxalate.
However, since a plating solution contains other salts such as complexing agent, additives, counter anion of copper ion, etc. in large amounts, the precipitate of potassium oxalate can be formed at a concentration lower than its saturation solubility in pure water.
Further, in xe2x80x9cHyomen Gijutsu (Surface Technology)xe2x80x9d, Vol.42, No.9, Page 913-917 (1991), paragraph of experimental method and in Table 1 of xe2x80x9cThe 6th national convention Record of Purinto Kairo Jissou Gakkai (Japan Institute of Printed Circuit)xe2x80x9d, Pages 101-102, there is mentioned that NaOH was used for the purpose of making the pH value of plating solution alkaline (pH=12.5) at the time of preparing the plating bath, so that precipitation of sodium oxalate, in an amount corresponding to the sodium ion concentration present in the plating solution, unavoidably took place in the initial stage of preparing the plating solution, which caused various problems such as formation of plating void, etc.
In Examples 22, 24, 25 and 30 of JP-A-61-183474, there is mentioned an electroless copper plating solution containing glyoxylic acid as a reducing agent and no substantial quantity of sodium. Such plating solutions are considered improved in stability and longer in lifetime as compared with plating solutions having a similar formulation. This is attributable to that potassium oxalate is higher than sodium oxalate in solubility and that the period of time having passed from the start of plating to the first occurrence of floatation and precipitation of oxalate in the plating solution is longer.
However, there is a problem that although the solubility of potassium oxalate is really higher than that of sodium oxalate, solubility of potassium oxalate is far lower than that of formate which is an oxidized product formed from formaldehyde, so that the lifetime of plating solution, namely the period of time having passed from the start of plating process to the first floatation and precipitation of potassium oxalate in the plating solution, is shorter than that in the case of using formaldehyde.
Even in the case of an electroless copper plating solution containing no substantial quantity of sodium, the lifetime as a plating solution is so short as a half or less as compared with the lifetime of general electroless copper plating solutions using formaldehyde as a reducing agent.
Such a short lifetime of plating solution brings about various demerits such as an increased cost of materials used for plating solution, an increased payroll needed for refreshing the plating solution, and an increased quantity of waste due to the short life-time of plating solution.
In conclusion, in the prior electroless copper plating solution using KOH as a pH adjusting agent, the Cannizzaro reaction-suppressing effect drops to only 15% five hours after start of plating process, and such a low suppressing effect is insufficient for an electroless copper plating solution which must be used over a long period of several tens to several thousands hours.
On the other hand, the method of stabilizing a plating solution by addition of methanol only is not suitable because this method does not suppress the Cannizzaro reaction itself, and one of the factors determining the lifetime of electroless copper plating solutions using glyoxylic acid as a reducing agent is the accumulation of oxalic acid due to Cannizzaro reaction.
For the above-mentioned reasons, the technique of electroless copper plating using glyoxylic acid as a reducing agent has not been adopted extensively as an industrial process.
It is an object of this invention to provide an electroless copper plating solution using glyoxylic acid as a reducing agent which is small in the reacting quantity of Cannizzaro reaction, does not largely cause the precipitation of the salt accumulated in the electroless copper plating solution due to the plating reaction and Cannizzaro reaction, and is stably usable over a long period of time.
It is another object of this invention to provide an electroless copper plating process which makes it possible to carry out plating in a high stability for a long period of time by the use of an electroless copper plating solution using glyoxylic acid as a reducing agent.
It is yet another object of this invention to provide a process for producing a circuit board which can be plated with an electroless copper plating solution using glyoxylic acid as a reducing agent to form a plating film keeping stable over a long period of time.
In order to solve the problems mentioned above, this invention provides an electroless copper plating solution comprising copper ion, a complexing agent for copper ion, a reducing agent for copper ion and a pH adjusting agent, wherein said reducing agent for copper ion is glyoxylic acid or a salt thereof, said pH adjusting agent is potassium hydroxide, and said electroless copper plating solution contains at least one member selected from the group consisting of metasilicic acid, a salt of metasilicic acid, germanium dioxide, a salt of germanic acid, phosphoric acid, a salt of phosphoric acid, vanadic acid, a salt of vanadic acid, stannic acid and a salt of stannic acid in an amount of 0.0001 mol/L or more.
Further, in order to achieve the object mentioned above, this invention provides an electroless copper plating solution comprising copper ion, a complexing agent for copper ion, a reducing agent for copper ion and a pH adjusting agent, wherein said reducing agent for copper ion is glyxoylic acid or a salt thereof, said pH adjusting agent is potassium hydroxide, and said electroless copper plating solution contains at least one member selected from the group consisting of a primary amine, a secondary amine and methanol in an amount of 0.001 mol/L or more.
If desired, said electroless copper plating solution may additionally contain at least one member selected from the group consisting of 2,2xe2x80x2-bipyridyl, 1,10-phenanthroline, 2,9-dimethyl-1,10-phenanthroline, polyethylene glycol and polypropylene glycol.
In any one of the electroless copper plating solutions mentioned above, the quantities of each of sodium ion, iron ion, nitrate ion and nitrite ion can be 10 mg/L or less.
In order to achieve the above-mentioned another object of this invention, this invention further provides an electroless copper plating process using any one of the electroless copper plating solutions mentioned above which comprises continuously circulating and filtering the plating solution after a preparation of a plating bath but prior to a plating treatment of a body to be plated.
The period of time T required for continuously circulating and filtering the plating solution after a preparation of plating bath but prior to a plating treatment of a body to be plated preferably satisfies the following formula:
Yxc2x7T greater than 3V
wherein V denotes a quantity of the plating solution and Y denotes a quantity of circulation per unit time.
In order to achieve the above-mentioned yet another object of this invention, this invention provides a process for producing a circuit board using any one of the above-mentioned electroless copper plating solutions which comprises continuously circulating and filtering the plating solution after a preparation of a plating bath but prior to a plating treatment of a base board.
In this case, too, the period of time T required for continuously circulating and filtering the plating solution after a preparation of plating bath but prior to a plating treatment of a body to be plates preferably satisfies the following formula:
Yxc2x7T greater than 3V
wherein V denotes a quantity of the plating solution and Y denotes a quantity of circulation per unit time.
In order to achieve the above-mentioned yet another object of this invention, this invention provides a process for producing a circuit board which comprises forming a copper film by the use of any one of the above-mentioned electroless copper plating solutions and thereafter electroplating by using said copper film as a seed film for electro plating.
Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention.